There are two kinds of hematopoiesis: one is the transient primitive hematopoiesis (embryonic hematopoiesis) that functions only during the embryonic stage, and the other is the definitive hematopoiesis (adult hematopoiesis) that contributes to lifelong hematopoiesis. Research by Medvinsky et al. (Medvinsky et al., Cell 86:897-906, 1996; Cumano et al., Cell 86:907-916) revealed that, in contrast to primitive hematopoiesis that develops within the yolk sac on around embryonic day 9, definitive hematopoiesis is initiated within the AGM (Aorta-Gonad-Mesonephros) region on around embryonic day 10. Furthermore, regarding the origin of hematocytes, various studies have suggested that definitive hematopoiesis originates from hemangioblasts, thought to be precursor cells common to hematopoietic cells and vascular endothelium cells.
While the mainly accepted view was that hemangioblasts, which are the origin of definitive hematopoiesis, exist in the AGM region, Yorder et al. argued against the existing theory and demonstrated that hemangioblasts, which may contribute to definitive hematopoiesis, also exist in the yolk sac (Yoder et al., Immunity 7:335-344, 1997). Therefore, it is now generally accepted that the surrounding environment is important for the differentiation of hemangioblasts to hematopoietic cells.
Thus, while the origin of hematopoietic cells and the site of development have been gradually elucidated by phenomenological research, the molecular mechanism of hematopoietic development remains unclear. The isolation of a novel molecule involved with primitive hematopoiesis is thought to be an important step for the development of unprecedented drugs associated with hematological disorders.